The present invention relates to a current sensor according to the compensation principle, in which the magnetic field produced by a primary winding through which the current to be measured flows is compensated by the compensation current in a secondary winding, and in which for the controlling of the compensation current at least one sensor that is influenced by the magnetic field acquires deviations from the zero current and supplies this measurement value to a driver circuit for the production of the compensation current, whereby at the output of the driver circuit the secondary winding is connected in series to a terminating resistance, and a voltage that is proportional to the current to be measured is present at the terminating resistance.
A current sensor of this type according to the compensation principle is for example known from European Reference EP 356 248 and from European Reference EP 691 544, and is shown in FIG. 5 of the drawing. The current i.sub.1 that is to be measured thereby flows through the primary winding 1 of a current transformer, which for example comprises a magnet core 2 as well as a sensor 3 that measures the magnetic flux in the magnet core 2.
The sensor 3 consists for example of a transformer driven to saturation, with a rectangular magnetization characteristic. The output voltage of the sensor 3 is processed in a subsequently connected evaluation circuit 4; a driver circuit 5 is in turn connected downstream therefrom. The output of the driver circuit 5 is connected with a reference potential G via the secondary winding 6 of the current transformer and a terminating resistance 7.
The current to be measured now produces, via the primary winding 1, a magnetic flux in the magnet core 2, which flux is acquired by the sensor 3. The evaluation circuit 4 connected after the sensor 3 supplies a signal--which is dependent on the magnitude and direction of the magnetic field in the magnet core 2--to the driver circuit 5, which signal drives a compensation current i.sub.2 through the secondary winding 6. The compensation current i.sub.2 is oriented in such a way that its magnetic field compensates the magnetic flux in the magnet core 2. The current in the secondary winding 6 is altered by the sensor 3 in connection with the evaluation circuit 4, the driver circuit 5, and the secondary winding 6 until the magnetic field in the magnet core 2 goes to zero. The current i.sub.2 in the secondary winding 6 is thereby a measure for the momentary value of the current i.sub.1 to be measured in the primary winding 1, whereby both direct and alternating currents can be acquired. In addition, the current i.sub.2 flows via a terminating resistance 7 at which the output voltage Ua of the current sensor falls off, which output voltage thereby corresponds in magnitude and phase position to the current i.sub.1 to be measured in the primary winding 1.
The maximum measurable current i.sub.1max of the compensation current sensor is thereby: EQU i.sub.1max =w.sub.2.multidot.(U.sub.V -U.sub.B)/R.sub.1 +R.sub.a),
whereby w.sub.2 is equal to the number of secondary windings, U.sub.V is equal to the supply voltage, U.sub.B is equal to the voltage drop in the driver stage 5, R.sub.i is equal to the internal resistance of the secondary winding 6 and R.sub.a is equal to the resistance value of the terminating resistance 7.
The known compensation current sensors operate predominantly with a terminating resistance that is connected in series with the secondary winding against a reference potential. By this means, for the compensation of a positive current a positive (or, respectively, negative) supply voltage is required, and for the compensation of a negative current a negative (or, respectively, positive) supply voltage is required. As a consequence, a bipolar voltage supply must be provided. If no such voltage supply is available, two bipolar voltages can be obtained from a unipolar voltage source by voltage halving; however, the individual voltage values thereof are thereby also halved. However, due to the halved voltage values the maximal measurable voltage is also reduced according to the above equation.